The goal of this investigation is to provide a complete thermodynamic description of protein-protein and protein-effector interactions in the allosteric enzyme aspartate transcarbamylase. The direct calorimetric determination of enthalpies is emphasized. The results of this study will make it possible to formulate rigorous thermodynamic criteria against which models of the catalytic and regulatory mechanisms can be tested. We have completed an analysis of the binding of nucleoside triphosphates to the native enzyme. An analogous study with the regulatory subunit is underway, and an analysis of the binding of N-phosphonacetyl-L-aspartate, carbamyl phosphate, and carbamyl phosphate and succinate to both the native enzyme and the catalytic subunit is in progress. In the next grant period we will study: (a) the binding of other substrates analogs, (b) subunit reassociation, (c) protein-protein interactions in the catalytic and regulatory subunits, and (d) the ionization of the native enzyme and its subunits, in the presence and absence of effectors. All of these studies will eventually be extended to mutant and chemically modified enzymes with altered allosteric properties.